An electrolytic capacitor is configured by using a valve metal having an insulating oxide film formed on the surface of aluminum or tantalum etc. as the anodic electrode foil, using said oxide film layer as the dielectric, contacting the surface of this oxide film layer with an electrolytic solution that will be the electrolyte layer, and further placing an electrode foil ordinarily referred to as the cathode for collecting current.
The electrolytic solution for electrolytic capacitor is in direct contact with the dielectric layer as described above, and acts as a true cathode. In other words, the electrolytic solution is interpositioned between the dielectric and the collector cathode of the electrolytic capacitor, and the resistance of the electrolytic solution is serially inserted into the electrolytic capacitor. Accordingly, the property of the electrolytic solution will be a great factor in influencing the properties of an electrolytic capacitor.
In the conventional technology of electrolytic capacitors, it is common to use an electrolytic solution having high withstand voltage and small decomposition of electrode foil as the electrolytic solution for medium/high-voltage electrolytic capacitor in an attempt to stabilize properties at medium/high-voltage, wherein the electrolytic solution employs ethylene glycol as the solvent and azelaic acid, sebacic acid, 1-methyl-azelaic acid, 1,6-decanedicarboxylic acid, or a salt thereof as the solute.
In the meantime, a power factor improvement circuit has been recently integrated on the primary side of a power source as a countermeasure for high frequency, and with this, a high-frequency low-impedance electrolytic capacitor for medium/high-voltage that can withstand a large high frequency ripple current has been desired.
Conventionally, a method for reducing the impedance of an electrolytic capacitor at high frequency has been done by reducing the specific resistance of the electrolytic solution. A method of mixing a large amount of water into the electrolytic solution is known as a method for reducing the specific resistance of the electrolytic solution. (Patent Document 1)
However, when the electrolytic solution for medium/high-voltage electrolytic capacitor employed comprises a large amount of water, reliability as an electrolytic capacitor was reduced in high-temperature shelf test due to for example the oxide film of the anodic electrode foil for the medium/high-voltage electrolytic capacitor being dissolved by water in the electrolytic solution (hydration decomposition), the electric capacitance being increased compared to the initial property, and the withstand voltage being decreased.
A method for adding an additive that suppresses the hydration reaction between the electrode foil and water is known as such a method for slowing the hydration decomposition of the electrode foil. (Patent Document 2)    Patent Document 1: Japanese Published Unexamined Patent Application Publication No. 11-145004    Patent Document 2: Japanese Published Unexamined Patent Application Publication No. 2002-164260
However, in a medium/high-voltage electrolytic capacitor such as that in Patent Document 2, addition of water conventionally could not be allowed to be greater than 10 wt % in the medium/high-voltage electrolytic capacitor and thus impedance at high frequencies could not be sufficiently lowered, since the hydration reaction suppression effect of the additive against the electrode foil will become insufficient when the amount of water mixed is greater than 10 wt %. For this reason, a medium/high-voltage electrolytic capacitor that yields sufficient low-impedance property at high frequencies and has high reliability has not yet been proposed.
Accordingly, the object of the present invention is to provide a medium/high-voltage electrolytic capacitor which is high-frequency low-impedance and further having good reliability.